The present invention generally relates to video cassette recorders and, more particularly, to a tracking control apparatus for use in assemble recording, such as a still picture file system, in which a tape is intermittently transported to intermittently record a video signal.
In a so-called 8 mm video cassette recorder, a tracking servo is carried out without using a CTL (control) head. This servo system is what might be called an ATF (automatic track finding) system.
According to the ATF system, tracking pilot signals having four different frequencies f.sub.1 to f.sub.4 are recorded on the tracks in a superimposed state with a video signal and an audio FM (frequency modulated) signal by a rotary head. The magnetic tape on which the tracking pilot signals are recorded is transported at a predetermined speed and the recorded signal is reproduced by the rotary head. Then the reproduced pilot signal separated from the recorded signal is compared with a reference pilot signal and a rotary phase of a capstan motor is controlled on the basis of a magnitude of a beat component obtained from a compared output.
As shown in FIG. 1, for example, when a magnetic head 1 of channel 2 (CH-2) traces a track in which the tracking pilot signal (TPS) of frequency f.sub.2 is recorded, a reproduced signal from a track 3 on the magnetic tape 2 is mixed with tracking pilot signals of frequencies f.sub.1 and f.sub.3 of the adjacent tracks 4 and 5 in addition to the tracking pilot signal of frequency f.sub.2. A reference pilot signal of frequency f.sub.2 generated within a reproducing apparatus (not shown) is supplied to a balanced modulator circuit (not shown) which derives beat components of frequencies 16 kHz and 47 kHz. If the rotational speed of the capstan is decreased by some event so as to shift the head to the track in which the tracking pilot signal of frequency f.sub.2 is recorded, the beat component of 16 kHz is increased and the beat component of 47 kHz is decreased. Conversely, if the rotational speed of the capstan is increased, the beat component of 16 kHz is decreased and the beat component of 47 kHz is increased. Therefore, if the capstan is servo-controlled such that the beat components of 16 kHz and 47 kHz may become equal to each other, the head of CH-2 correctly traces the track in which the tracking pilot signal of frequency f.sub.2 is recorded.
Japanese Patent Laid-Open Gazette No. 59-65962, incorporated herein by reference, describes in detail this type of ATF control apparatus.
In the playback mode utilizing such ATF servo., the center of the head position is made coincident with the center of the track. In that case, if the width of the head is different from the track width after the recording, the optimum position of the head relative to the track is fluctuated in the recording mode and in the reproducing mode. More specifically, during the SP (standard playback) mode, the track width is 20.5 .mu.m while the width of the head is 25 .mu.m. As a result, during the recording mode, as shown in FIG. 2, a lower edge of a head 1R must correspond with a lower edge of a track; and during the reproducing mode, a center of a head 1P must correspond with a center of a track. For example, when the recording mode is changed to the recording pause mode and is again changed to the recording mode, in the forward (FWD) running mode (in that case, the ATF servo system is set in the playback mode) after the recording pause mode is released, the lower edge of the head must correspond with the lower edge of the track in the same way as in the recording mode. For this purpose, in the forward running mode, the ATF servo control method is changed to prevent a noise from being produced at the connection point of events. An outline of this operation will be described with reference to a timing chart forming FIGS. 3A-3D.
Reference numerals [1], [2], [3] and [4] of FIGS. 3A-3D represent frequencies of reproduced ATF pilot signals. FIG. 3A shows a timing chart of an RF switching pulse. FIG. 3B shows a timing chart of a reference ATF pilot signal. FIG. 3C shows a timing chart of an ATF sampling pulse. FIG. 3D shows a beat signal of 16 k/47 kHz. For example, during a period in which the magnetic tape is transported in the forward direction in the recording pause mode, the following operation is carried out in units of 4 fields as shown in FIG. 3A. In the portions shown by A in FIG. 3A, only during the period corresponding to the shift amount are the ATF reference pilot signal and the beat signal of frequency 16 k/47 kHz made the same as those of the ATF error detecting period of the next field (track). For example, when the head reproduces the magnetic tape at its field (track) in which the reproduced frequency is f.sub.1, if the frequency of the ATF reference pilot signal is selected to be f.sub.2 only during the period whose duration corresponds to the necessary shift amount, a beat component of frequency 16 kHz (f.sub.2 -f.sub.1) occurs, and the beat signal of frequency 16 k/47 kHz is at a high (H) level. Thus, the AFT error increases and hence the position of the head relative to the track is advanced and the lower edge of the head corresponds with the lower edge of the track. This shift amount is expressed as: ##EQU1##
As described above, according to the lower edge tracking operation of the 8 mm VCR, the track is shifted slightly from the start of the movement of the tape to the end of movement. The above-described technique cannot be utilized in the time lapse recording, and the lower edge of the head cannot correspond with the lower edge of the track accurately according to this tracking servo operation because of the predictive control operation.
More specifically, the 8 mm VCR utilizes a capstan made of an oil-less metal so that, upon reproduction during which the capstan is normally rotated, an oil film is substantially uniformly formed on the surface of the capstan by a centrifugal force. This forms a lubricant which provides smooth rotation of the capstan. However, when the capstan is revolved intermittently, the effect of the above-described lubricant cannot be expected, which causes a torque ripple to occur in the capstan.
Furthermore, due to the change of the rotational interval of the capstan, a rotational angular error of the capstan is produced even by the same drive voltage. For example, when a capstan motor is once actuated and the next drive voltage is supplied to the capstan motor while extra energy still remains in the capstan motor or when the next drive voltage is applied while the energy of the capstan motor by the former drive voltage is decreased to be zero, the inertia of the capstan becomes different so that the initial condition of the revolution of the capstan is changed. That is, an ATF error is changed with the driving speed of the capstan motor. Therefore, different ATF errors were measured and examples thereof will be described with reference to FIGS. 4A-4C and FIGS. 5A-5C.
Throughout FIGS. 4A-4C and FIGS. 5A-5C, a curve 6 represents a signal which revolves the capstan, and a curve 7 represents an ATF error voltage. FIGS. 4A-4C illustrate the mode in which the capstan is actuated, and FIGS. 5A-5C illustrate the ATF voltages in the auto stop mode.
As is clear from these curves 6 and 7, the ATF error voltages fluctuate considerably. Accordingly, when the revolution of the capstan is controlled on the basis of these ATF error voltages, the ATF error voltages do not always become uniform due to the different kinds of video cassette recorders, and due to the load condition of the capstan.